1. Field of the Invention
The present invention relates to an organic light-emitting element having an anode, a cathode and a layer including an organic compound, which emits light when an electronic field is applied thereto (hereinafter referred to as an ‘electroluminescent layer’), and a light-emitting device using such an organic light-emitting element. In addition, the invention relates to an electronic appliance mounted with the light-emitting device having an organic light-emitting element as its component.
2. Description of the Related Art
In recent years, a light-emitting device having an EL element as a self-luminous element has been actively developed. This light-emitting device is also called an organic EL display or an organic light-emitting diode. Such a light-emitting device has advantages in that low-voltage and low-power-consumption drive can be performed with high response speed which is suitable for displaying moving images. Therefore, the light-emitting device has been attracting attention as a next-generation display such as a new-generation portable phone or portable information terminal (PDA).
The light-emitting element has a pair of electrodes (an anode and a cathode) and an electroluminescent layer interposed therebetween. It is said that light emission is obtained when a hole injected from the anode and an electron injected from the cathode upon application of an electronic field to the both electrodes are recombined in the luminescent center of the electroluminescent layer, thereby a molecular exciton is formed to release energy in returning to the ground state.
The organic EL display having a light-emitting element is a self-luminous type differently from a liquid crystal display which requires a backlight, thus it has no problem with regard to the viewing angle. That is, the organic EL display is more suitable for a display for outdoor use than a liquid crystal display, and various applications thereof have been proposed.
A driving method of the organic EL display can be roughly classified into Direct Current (DC) drive and Alternate Current (AC) drive.
The present applicant has disclosed Patent Document 1 in which AC drive is used in an active matrix display device having an EL element, and disclosed Patent Document 2 in which an EL element is used for the AC drive.
[Patent Document 1] Japanese Patent Laid-Open No. 2001-222255
[Patent Document 2] Japanese Patent Laid-Open No. 2004-95546
When AC drive is performed, an AC signal (signal of which voltage level is inverted in regular cycles) is applied to a pair of electrodes of an EL element. In the case where the EL element has a simple structure such as a stacked-layer structure of a first electrode, a second electrode and a layer including an organic compound interposed therebetween, light emission is obtained only in a half cycle when an AC signal is applied thereto due to the rectifying function. Accordingly, in order to obtain about an equal amount of light to the case of performing DC drive in which DC voltage is applied to the EL element, power consumption is increased.